Bioactive glass was first invented by Hench and collaborators at the University of Florida in 1971 (L. L. Hench, R. J. Splinter, W. C. Allen, J. Biomed. Mater. Res. Symp., 1971, (2):117˜141). Bioactive glass not only possesses excellent bioactivity, but also has favorable bonding to bone and soft tissue. Furthermore, it holds long-term security and stability when being implanted into body as an artificial bone replacement material. Therefore, bioactive glasses have great potential in dental, orthopedic, otology, and restoration applications (L. L. Hench, Bioceramics, J. Am. Ceram. Soc., 1998, (81):1705˜1728). At present, bioactive glasses have received the approval from the U.S. Food and Drug Administration (FDA). However, the inferior mechanical properties of bioactive glasses, such as high brittleness and low toughness, limit their usages. It is one of the main methods to improve medical implants' bioactivity by depositing bioactive glass coatings onto Ti and Ti-based alloys. It integrates the superior mechanical strength of the Ti and Ti-based alloys and the excellent bioactivity, biocompatibility, osteoconductivity and osteoinductivity of the bioactive glasses. Hence, bioactive glass coatings were widely used in clinical applications, such as bone replacement and restoration.
Bioactive glass coatings were deposited on Ti and Ti-based alloys by various methods, including: powder metallurgy sintering, acidic and alkali treatment, laser cladding, pulsed laser deposition, magnetron sputtering, electrophoretic deposition, sol-gel, air plasma spraying, etc. Among them, sol-gel and air plasma spraying are the most widely used and researched methods at present. In the sol-gel method, the organic alkoxide was firstly hydrolyzed, and then the inorganic salt solution was added to form the sol. After being aged for a period of time, the sol was coated on the pretreated Ti and Ti-based alloy substrates. The sol coated substrate was dried at a certain temperature, and then the coating-drying process was repeated for many times. Finally, the coated substrate was sintered at a certain temperature to form the bioactive glass coating (L. D. Piveteau, M. I. Girona, L. Schlapbach, et al., Materials Science: Materials in Medicine, 1999, (10): 161-167). Air plasma spraying is one kind of the thermal spray methods. In the air plasma spraying method, there are stringent requirements on the preparation of the powder feedstocks. At first, the bioactive glass powders or blocks were often prepared through melt-and-quench process or sol-gel process. Then, the powders or blocks were crushed, ball milled and sieved to obtain the feedstocks for air plasma spraying. Next, the obtained powder feedstocks were added into the special feeding device of the plasma spray system. The powder feedstocks were transported through the tube under pressure, to the powder nozzle near the plasma spray gun. Finally, the powder were spurted into the plasma flame directly, rapidly melted in the high temperature plasma flame, and deposited on the Ti and Ti-based alloys to form the bioactive glass coatings (T. M. Lee, E. Chang, et. al., Surface and Coatings Technology, 1996, (79):170-177).
Bioactive glass coatings prepared by the sol-gel method have the following advantages: low reaction temperature, easily controlled reaction process, high uniformity and purity, exact stoichiometric composition, readiness for modification, wide range of doping composition, high bioactivity. However, this method also has the following shortcomings: long process time, thin coating thickness (usually about 10 μm), easy to crack, low bonding strength between the coating and substrate, complicated procedures, low productivity, unsuitable for large-scale production. Currently, air plasma spraying is one of the most widely used methods domestically and internationally, and is also the only successfully used technique in commercial application so far. The bioactive glass coatings prepared by air plasma spraying possesses high deposition efficiency, controllable thickness and high bonding strength between the substrate and the coating. However, there are stringent requirements on the preparation of the bioactive glass powder feedstocks. The feedstocks were commonly prepared through melt-and-quench process. Firstly, all the oxide constituents were mixed together and melted at high temperature. Then, the mixture was quenched to form bioactive glass blocks or powders. Finally, the blocks or powders were crushed, ball milled and sieved to obtain the feedstocks for air plasma spraying. Since it is complicated and time-consuming, the process has low productivity and high cost.